1. Field of the Invention
This invention relates to a carbonaceous negative electrode material for nonaqueous secondary batteries, a process for producing the negative electrode material, and a nonaqueous secondary battery containing the negative electrode material. More particularly, it relates to a carbonaceous material having a specific micropore size distribution and amount which is particularly suited as a negative electrode material of nonaqueous secondary batteries, a process for producing the same, and a nonaqueous secondary battery typified by a lithium ionsecondary battery, which is suitable for use in compact and light electronic equipment, for use in an electric car power load leveling, both of which require high capacity and high energy density.
2. Description of the Related Art
The demand for rechargeable secondary batteries has been increasing in pursuit of reduction of size and weight and improvement of function of electronic equipment and high attention paid to the electric car caused by an environmental pollution. Clean nonaqueous batteries, such as nickel-hydrogen or lithium batteries, have been attracting attention as batteries meeting the demand, taking the place of lead or nickel-cadmium batteries from the viewpoint of power saving and environmental conservation. In particular, lithium ion secondary batteries have been gaining weight from their lightness and high voltage and been put to practical use. Lithium ion secondary batteries developed in the early years used metallic lithium in the negative electrode, which turned out disadvantageous in that a dendrite grows from metallic lithium to cause an internal short-circuit. Then materials capable of intercalating and deintercalating lithium ion have been developed in place of metallic lithium. Along this line, carbonaceous materials, such as coke, and graphite materials, such as natural graphite, have been used mainly (see JP-A-2-90863, JP-A-1-221859, and JP-A-63-121257, the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d).
Of the carbonaceous materials and the graphite materials, the carboneous materials prepared by calcining at relatively low temperatures, for example 1500xc2x0 C. or lower, can be seen as promising. as high-capacity and low-cost negative electrode materials. On the other hand, although carbonaceous materials are relatively inexpensive and safe and have a high charge capacity in the initial stage of use, their charge and discharge capacity in the later stage is about ⅔ of the theoretical capacity density of carbon-graphite materials (372 mAh/g). Batteries containing the conventional carbonaceous materials are not satisfactory in charge and discharge capacity and efficiency, leaving room for improvement.
The inventors of the present invention previously found that the high initial charge capacity of a carbonaceous material owes to its volatile matter, that the volatile matter which is released on heating is partly carbonized to interfere with the manifestation of the capacity of the carbonaceous material, and that the volatile matter can be removed efficiently by pulverizing the carbonaceous material followed by heating at a high temperature (calcination) thereby to suppress remaining of the carbonized volatile matter. These findings were filed for a patent (JP-A-8-287911).
As a result of further study, the inventors have reached the findings that the remained volatile matter, especially free organic high-molecular substances, which has been a main cause of the above-mentioned problem can be removed efficiently by subjecting fine carbonaceous powder to specific heat treatment to obtain a carbonaceous material having a controlled micropore size distribution and thereby exhibiting improved charge and discharge capacity and efficiency when used as a negative electrode material in a nonaqueous secondary battery.
Completed based on the above findings, the present invention provides a carbonaceous negative electrode material for nonaqueous secondary batteries having (i) not less than 2xc3x9710xe2x88x924 cc/g of pores having a diameter of smaller than 8 xc3x85, (ii) not more than 15xc3x9710xe2x88x924 cc/g of pores having a diameter of 8 to 18 xc3x85 and (iii) not more than 1xc3x9710xe2x88x924 cc/g of pores having a diameter not larger than 20 xc3x85.
The present invention further provides a nonaqueous secondary battery containing the carbonaceous material as a negative electrode material.
The present invention furthermore provides a process for producing a carbonaceous negative electrode material for nonaqueous secondary batteries comprising heating a particulate carbonaceous material having an average particle size of not greater than 100 xcexcm at a temperature of 250 to 650xc2x0 C. in an inert gas atmosphere and then heating (calcining) the particles at a temperature of 700 to 1500xc2x0 C. in an inert gas atmosphere (hereinafter referred to as a first process), or heating a particulate carbonaceous material having an average particle size of not greater than 100 xcexcm at a temperature of 50 to 400xc2x0 C. in an oxidative atmosphere and then heating (calcining) the particles at a temperature of 700 to 1500xc2x0 C. in an inert gas atmosphere (hereinafter referred to as a second process).
In the second process, the heating at 50 to 400xc2x0 C. in an oxidizing gas atmosphere can be preceded and/or followed by a heat treatment at a temperature of 250 to 650xc2x0 C. in an inert gas atmosphere (hereinafter referred to as a third process).
According to the present invention, a negative electrode material for nonaqueous secondary batteries having a high capacity and a high efficiency can be obtained with ease from inexpensive carbonaceous raw materials.